1. Field of the Invention
The present invention is directed to an x-ray computed tomography (CT) apparatus of the type having means for modulating the x-ray power of an x-ray source that is displaceable relative to an examination subject, and a detector system for the x-rays emanating from the x-ray source.
2. Description of the Prior Art
An x-ray CT apparatus of the above type is disclosed, for example, in U.S. Pat. No. 5,379,333, German OS 195 27 518, German OS195 32 535, German OS 28 15 347 and German OS 198 07 639.
An x-ray CT apparatus generally has an x-ray source that directs a collimated, fan-shaped x-ray beam through the examination subject, for example a patient, onto a single row or multi-row bank of detectors of the detector system. The source and, dependent on the model of x-ray CT apparatus, the detector system as well are mounted on a gantry that rotates around the examination subject. A support table for the examination subject can be moved through the gantry. The position from which the x-rays penetrate the examination subject and the angle at which the x-rays penetrate the examination subject are constantly varied as a result of the rotation of the gantry. Each detector of the detector system produces a signal that represents a measure of the overall transparency of the body of the examination subject for the x-rays emanating from the x-ray source proceeding to the detector. The set of output signals of the detectors of the detector system that is acquired for a specific position of the x-ray source is referred to as a projection. A scan comprises a set of projections that were acquired at various positions of the gantry and/or various positions of the support table. The x-ray CT apparatus registers a number of projections during a scan covering a full revolution of the gantry by 360xc2x0 around the examination subject in order to be able to construct a two-dimensional tomogram of a slice of the body of the examination subject. More recent CT apparatus simultaneously register a number of slices by employing detector systems having a number of rows of detectors. For each projection, a detector referred to as monitor or reference detector measures the unattenuated intensity of the x-radiation.
There are two systematically different methods for registering the data required for producing CT images of three-dimensional regions of an examination subject.
In conventional scanning, the data are registered during a complete revolution of the gantry and one slice is thus scanned while the examination subject is in a fixed position. The examination subject is moved into a new position between the scanning of successive slices, and the next slice is scanned. This procedure continues until all slices defined by the examination have been scanned.
In a spiral scan, the gantry rotates together with the x-ray source around the examination subject while the support table and the gantry are continuously displaced relative to one another in the direction of the longitudinal axis of the support table. The x-ray tube thus describes a spiral (helical) path with respect to the examination subject until the volume defined by the examination has been scanned. Images of individual slices are then calculated from the spiral data acquired in this way.
In each of these two scan types, the image quality is degraded by quantum noise. The radiation intensity of the x-rays omitted by the x-ray source therefore must be high enough for each projection in order to assure that the minimum radiation intensity of the x-rays emerging from the body of an examination subject and proceeding to a detector will be higher than the noise level.
In known x-ray CT apparatuses with modulation of the x-ray power, the radiation intensity, i.e. the photon flux, is suitably modified as a function of the angular position of the gantry during the scanning of the examination subject in order to process the areas of the examination subject that contain high beam attenuation with the required, high radiation intensity when registering projections, as well as to process the areas of the examination subject that contain lower beam attenuation with correspondingly lower radiation intensity when registering projections, so as to expose the subject to a lower radiation stress.
Such x-ray CT apparatuses, for example, require two orthogonal tomograms (U.S. Pat. No. 4,174,481) or xe2x80x9cScout Viewsxe2x80x9d (U.S. Pat. No. 5,379,333) in order to be able to acquire the information necessary for modulation of the x-ray power with respect to the curve of the maximum beam attenuation value of the individual projections as function of the angular position of the gantry, i.e. with respect to the angle attenuation profile of the examination subject.
To that end, for example, an angle attenuation profile approximated to the actual conditions can be identified from the attenuation information of each line of the tomogram or from the xe2x80x9cScout Viewsxe2x80x9d.
The corresponding modulation of the x-ray power ensues by setting the radiation intensity of the x-rays corresponding to the desired x-ray power by modifying the tube current of the x-ray source, namely by influencing the filament current of the glow cathode. Such a setting of the tube current, however, has relatively high inertia (delay) because of the limited heating and cooling rate of the glow cathode, and thus the modulation of the x-ray power can only unsatisfactorily follow angle attenuation profiles having rapid changes of the beam attenuation. Due to an inadequate matching of the modulation of the x-ray power to the angle attenuation profile, additional, even non-uniform image noise therefore occurs.
An object of the present invention is to provide an x-ray CT apparatus of the species initially described wherein the modulation of the x-ray power is also capable of following rapid changes in the attenuation profile.
This object is inventively achieved in an x-ray CT apparatus having an x-ray source which is displaceable relative to an examination subject and a detector system for the x-rays emanating from the x-ray source, with projections for different positions of the x-ray source, being registered and wherein the x-ray source emits x-rays in one or more pulses during the registration of a projection, and wherein the x-ray power is modulated by setting the duration of the pulses dependent on an attenuation profile of the examination subject for the individual projections such that the mAs product supplied on average for each projection at least qualitatively corresponds to the curve of the attenuation profile.
In a traditional scan, wherein the x-radiation source rotates around the examination subject, and in a spiral scan, where at least the x-ray beam rotates around the subject while a relative displacement between the x-ray source/detector system and the examination subject in the direction of the axis of the rotation is undertaken, the attenuation profile is in the form of an angle attenuation profile. When, by contrast, a scan is undertaken wherein the rotation of the x-ray beam does not take place and only a relative displacement between the x-ray source/detector system and the examination subject in the direction of the axis of the rotation ensues, the attenuation profile is in the form of an axial attenuation profile, i.e. it reflects the curve of the beam attenuation of the examination subject in the direction of the axis of rotation.
The invention thus is suitable for conventional scanning, spiral scanning and for the registration of so-called topograms.
In the invention, thus, the modulation of the x-ray power does not ensue in such a way that the radiation intensity of the x-radiation is varied via the filament current of the glow cathode but instead the x-ray source emits x-rays in a number of pulses during the registration of a projection, the duration of these pulses being modified for influencing the average x-ray power applied for the registration of the respective projection. By contrast to the influencing of the x-ray power via the filament current of the glow cathode, a modification of the average x-ray power by pulsing the x-radiation is possible practically without inertia, so that the modulation of the x-ray power is capable of following rapid changes in the attenuation profile.
It is advantageous that the radiation intensity need not be modified during a scan; rather, the same radiation intensity can be present in all pulses of a scan, as is preferable, since the momentarily existing x-ray power need not be modified but only the average x-ray power effective during the registration of a projection need be modified by a corresponding setting of the duration of the pulses. At the same time, this offers the advantage that work can always be carried out with a radiation intensity that is adequately high for a good signal-to-noise ratio, possibly even the maximum radiation intensity of the x-ray source, without exposing the subject to an unnecessarily high radiation stress. This is because of the inventive modification of the duration of the pulses in conformity with the attenuation profile. The pulse duration is the determining feature, with a given number of pulses and given tube current, for the mAs product.
Since, due to the pulsing of the x-radiation, the x-ray source is not in operation during the entire time required for acquiring a projection, an improved image resolution is also achieved since, due to the shorter overall time during which the x-ray source is generally active for the registration of a projection, a reduction of the azimuthal smear of the measured values is achieved in conventional scans as well as in spiral scans, this being particularly true for subject regions at greater distances from the rotational axis.
The attenuation profile can be determined in a known way before the actual examination on the basis of topograms or scout views; however, the attenuation profile also can be pre-calculated from projections acquired in the past, using suitable algorithms during the examination.
In a preferred embodiment of the invention, the inventive x-ray CT apparatus has an x-ray source with a vacuum housing in which an anode and an electron emitter for generating an electron beam are accepted, the electron beam striking the anode for generating x-rays, and having a control electrode allocated to the electron emitter, for example a Wehnelt cylinder or a grid, which can be connected by the circuit for modulating the x-ray power to a blocking potential for interrupting the electron beam. The x-ray source is thus constructed analogously to a triode and allows an inertia-free pulsing of the x-rays.
In one version of the invention the circuit for modulating the x-ray power includes a switch for applying the blocking potential, this switch being combined with the x-ray source to form a unit. In this way, the connection between the switch and the control electrode is short, so that disadvantageous influences on the blocking behavior, and thus the pulsing of the x-rays as a result of high capacitance and inductance values of the lines located between the switch and the control electrode are precluded. The switch and x-ray source unit preferably is manufactured by either attaching the switch to the x-ray source, or disposing the switch in the inside of the vacuum housing of the x-ray source.
In a further embodiment of the invention that the circuit for modulating the x-ray power sets the duration of the pulses for each projection such that the mAs product required for adhering to a desired signal-to-noise ratio is achieved for the respective projection. It is assured in this way that a specific signal-to-noise ratio is achieved without administering an unnecessary x-ray dose to the examination subject.
In another embodiment of the invention x-ray CT apparatus has a monitor detector on which the x-rays emanating from the x-ray source are incident unattenuated, and the attenuation profile is calculated using the output signals of the detector system and the monitor detector.
In a preferred embodiment of the invention the circuit or components for the determination of the attenuation profile is/are displaceable relative to the examination subject together with the x-ray source and the detector system. In this case it is not necessary to transmit the data supplied by the detector system and required for the determination of the attenuation profile back and forth between a moving part and a stationary part. Moreover, the image computer is relieved of the task of calculating the attenuation profile, although it of course is possible for the image computer to undertake this task.